Wetting Stability of Aged Limestone in the presence of HPAM polymer
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- Master theses 
The Norwegian Petroleum Directorate required that a polymer field pilot should be performed in the Johan Sverdrup field in the second phase of the field development to confirm potential enhanced recovery potential at the field and collect valuable data. Polymer increases the viscosity of the aqueous phase, which gives a more favourable mobility ratio and improved volumetric sweep efficiency during production of high viscous oil or production in heterogeneous reservoirs. Although polymer injection is an established enhanced oil recovery method, it is still not established if injection of a hydrophilic polymer (HPAM) alter the wettability preference of oil-wet rock towards water-wet. This is important to answer because wettability largely dictates important flow functions. This experimental thesis evaluates change in wetting preferences during polymer injection using heterogeneous limestone rock core samples. A previously verified rock/brine/oil system was utilized to produce oil-wet rock surfaces, and systematic core scale tests have been used to describe important properties including: wetting preference stability in brine/oil systems, and the influence of hydrophilic polymers on wettability. Multiple Amott-Harvey cycles were used to determine wettability preferences and alterations in 29 core plugs. In-situ visualization by MRI, high-resolution CT and PET-CT assisted in the interpretation. Amott-Harvey cycles were first performed to determine wetting preference stability in brine/oil systems, where the core plugs were aged to slightly oil-wet conditions. Spontaneous imbibition of oil was recorded during the first three Amott-Harvey cycles and the wettability remained stable. Previous studies have not been performed beyond three cycles, but in this study two additional cycles were performed. During the 4th and 5th cycle, spontaneous imbibition of oil stopped, and small volumes of water imbibed spontaneously into some core plugs. Hence; the wettability changed from slightly oil-wet towards neutral or slightly water-wet conditions during completion of five subsequent Amott-Harvey cycles. When polymer was introduced to slightly oil-wet core plugs, oil imbibition ceased in the second Amott-Harvey cycle and polymer imbibition started in the third, i.e. polymer initiated a quicker change in wettability, and yielded stronger water-wet conditions than brine. Relative permeabilities of water and oil decreased when polymer was present in the pore volume. A larger decrease in relative permeability of water than the relative permeability of oil was observed both in water-wet and oil-wet cores. PET-imaging showed that oil and water displacement fronts propagated faster through a core plug when polymer was present in the pore volume, which indicated that the pore volume available for flow had been reduced by polymer injection. Blocking of narrow pores and pore throats by polymer, and adsorption of polymer layers on the pore walls was proposed as viable explanations for the observed behavior.